| Objective:There are two components of pain, sensory-discriminative and emotional-affective dimensions. It is possible that these components are mediated by separate neural systems. Human and animal studies before finding anterior cingulate cortex (ACC), especially, rostral ACC (rACC) plays an important role in pain-related emotional behavior. Early reports indicated that surgical ablation of the ACC and surrounding cortical tissue decreases pain-related conditioned place avoidance (CPA). Clinical and animal studies have inferred that acupuncture may ease the affective dimensions of pain, but its mechanism is not clear. The existing data show that acupuncture, a traditional therapeutic modality or electroacupuncture may induces the release of endorphins to block pain. So, we try to investigate if n-opioid receptor activation of ACC brain areas may reduce the CFA-induced CPA response in rats and something about mechanism of action.Methods:1. CFA-induced conditioned place avoidance (C-CPA).Male Sprague-Dawley rats (weight250-270g) were divided into three groups:(1) naive,(2) saline-injected, and (3) CFA-injected. The saline or CFA (0.08ml) was injected into the left hind paw,(n=6-8/group). The aim of this experiment was to determine whether the apparatus chambers produce neutral stimuli and whether CFA-induced spontaneous pain generated a negative affect that could be assessed with CPA response. If the time spent on CFA-paired chamber during the post-conditioning less than the time spent on pre-conditioning test, the model is sussessful.2. CFA-induced spontaneous pain generated a negative affect that could be assessed with CPA responses.In experiment2, rats were divided into five groups,(1) and (2) saline-injected/CFA-injected, saline was administrated in bilateral rACC areas,(3) CFA-injected,0.05/0.1/0.5or1(μg/μl) DAMGO were respectively administrated in bilateral rACC areas,(4) CFA-injected,10nmol CTOP+1(μg/μl) DAMGO in rACC area,(5) NS-injected,0.05/0.1/0.5or1(μg/μl) DAMGO were respectively administrated in bilateral rACC areas. The saline or CFA (0.08ml) was injected into the left hind paw. 3. Paw Withdrawal Thermal latency (PWL) in response to thermal stimuli was tested after the first day and second day of CPA rats behavior training.4. Through the rACC brain slices, we investigate μ-opioid receptor and NR2A/NR2B of NMD A receptor subunit with fluorescence localization method. The western-blot was used to detect of NMDA receptor and NR2A/NR2B subunits of phosphorylation level.Results:1. CFA could induce conditioned place avoidance (CPA)Naive (Pre286.1±38.62s VS Post297.6±75.02s, P>0.05, n=6) and NS-injected (Pre301.6±76.4s VS Post291.34±109.8s, P>0.05, n=7) showed no aversion to either chamber. CFA-injected rats (Pre304.1±39.8s VS Post156.5±80.2s, P<0.05, n=8) showed aversive responses to the CFA-paired chamber during the post-conditioning test, the model of CFA-induce conditioned place avoidance model was successful.2. When DAMGO was injected into rACC, it could inhibit CFA-induced affective responseNS-injected, NS was injected into rACC (Pre325.9±106.6s VS Post313.7±169.6s, P>0.05, n=9) showed no place avoidance to saline-paired chamber during the post-conditioning test. When CFA-injected, NS was injected into rACC (Pre331.4±48.66s VS Post146.7±129.4s, P <0.05, n=7) showed aversive responses to the CFA-paired chamber during the post-conditioning test. CFA-injected, different dose of DAMGO was injected into rACC, when0.05(μg/μl) DAMGO was injected into rACC (Pre348.7±48.62s VS Post334.6±109.2s, P>0.05, n=7), there are no difference between pre-conditioning day and post-conditioning day. As the dose of DAMGO increases, magnitude of CPA was no statistically significant. That showed low dose of DAMGO could inhibit of CFA-induced affective response. NS-injected, different dose of DAMGO was injected into rACC, there were still no palce avoidance. The magnitude of CPA was no statistically significant.3. When CTOP was injected into rACC, it could block DAMGO in rACCCFA-injected, we first inject CTOP, a mu-selective opioid receptor antagonist into rACC, then we inject DAMGO into rACC (Pre343.4±81.64s VS Post153.6±67.17s, P<0.05, n=6), CFA-induced CPA still take place.4. The CFA (0.08ml) was injected into one hind paw, PWL will change.PWL in response to thermal stimuli was tested after the first day and second day of CPA, when CFA-injected, PWL has significantly decrease (P<0.05). Therefore, NS-injected, PWL has no difference (P>0.05).5. DAMGO may ease the affective dimensions of CFA-induced but not change CFA-induced sensory-discriminative of pain When CFA-injected, PWL has significantly decrease whatever DAMGO was injected into rACC (P<0.05). On the opposite, when NS-injected, PWL has no difference (P>0.05).6. Within the rACC brain areas, mu-opioid receptors and NMDA receptor could expression togetherThrough the rACC brain slices, we investigate μ-opioid receptor and NR2A/NR2B of NMDA receptor subunit with fluorescence localization method. The results of western-blot were to follow-up perfect.Conclusion:1. CFA-injection into the hind paw induces nociceptive behaviors and conditioned place avoidance that could be a measurement of affective dimensions pain.2. DAMGO, a μ-opioid receptor activation may ease the affective dimensions of complete Freund’s adjuvant-induced and low doseage could be help. The high doseage did not induce preference.3. DAMGO may ease the affective dimensions of complete Freund’s adjuvant-induced is not rely on relieve sensory-discriminative of pain.4. Within the rACC brain areas, μ-opioid receptor may be regulate the function of neurons through NMDA receptor. |
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